The invention relates to a brake system for a motor vehicle comprising a master brake cylinder in which a primary piston for generating a brake pressure in a hydraulic brake circuit can be displaced according to actuation of a brake pedal, it being possible to displace the primary piston under the effect of a hydraulic servo pressure circuit in the master brake cylinder and the hydraulic servo pressure circuit comprising a pressure source for generating the servo pressure and a pressure accumulator for maintaining a minimum servo pressure.
Brake systems of this type are known from the prior art. Thus DE 195 42 656 A1, and corresponding U.S. Pat. No. 6,290,307 B1, both of which are incorporated by reference herein, for example discloses a brake system for a motor vehicle in which actuation of a brake pedal is detected by sensors. A servo pressure circuit is controlled by a control unit with the aid of the detected pedal actuation. A pressure source is provided in this servo pressure circuit and generates a servo pressure in the hydraulic servo pressure circuit. The pressurised hydraulic fluid is supplied by means of a manifold valve to the master brake cylinder. More precisely, the primary piston that can be displaced inside the master brake cylinder by means of the servo pressure for generating a brake pressure is also pressurised with a servo force, resulting from the servo pressure, in addition to the pedal actuation force. As a result, the primary piston can be effectively displaced inside the master brake cylinder without the entire force required for generating the brake force having to be applied by the brake pedal.
In order to be able to provide a sufficiently high servo pressure at any instant during operation, the servo pressure circuit of DE 195 42 656 A1 provides a pressure accumulator connected downstream of the pressure source. This pressure accumulator is directly coupled to the servo pressure circuit. Whenever the pressure source of the servo pressure circuit is active hydraulic fluid is conveyed from the servo pressure circuit into the pressure accumulator and stored therein. However, this has the effect that in a state in which there is a relatively low hydraulic pressure in the servo pressure circuit and in which the full power of the pressure source would be inherently required for generating a brake pressure inside the master brake cylinder for displacing the primary piston, a certain portion of the hydraulic fluid conveyed by the pressure source will also always be conveyed into the pressure accumulator owing to the fluidic connection of the pressure accumulator and pressure source. The efficiency and the response characteristic of the brake system suffers as a consequence, in particular as not all of the hydraulic fluid required for actuating the primary piston is conveyed from the servo pressure circuit into the master brake cylinder.